Activation of filaments



Dec. '18, 1928. 1,695,819

G. D. O'NEILL ACTIVATION 0F FILAMENTS Filed April 1, 1927 vAcvuM PUMP INVENTOR GEORGE D. 0 NE] LL ATTOR N EY Patented Dec. 18, 1928.

UNITED STATES PATENT OFFICE.

, GEORGE DEAN ONEILL, or BLOOMIIELD, NEW JERSEY, ASSIGNOR 'ro WESTINGHOUSE LAMP COMPANY, A cOnronArION or PENNSYLVANIA.

ACTIVATIQN F FILAMENTS.

Application filed April 1, 1927. Serial No. 180,252.

This invention relates to'a method and apparatus for activating thoriated tungsten filaments'for electron emission purposes and more particularly to the carbonization thereof for the purpose of converting the thorium oxide content thereof to thorium metal.

It is the. practice in the manufacture of thoriated tungsten filament to incorporate into the purified tungsten powder a small since it volatilizes from the tungsten during.

the heat treatments to which the material is subjected in manufacture. Various methods have been proposed for converting the thorium oxide to thorium metal upon completion of the filament, the most practical method being by employing a'carbonaceous reducing agent.

It has been proposed heretofore .to. heat the filament at a high temperature for a few minutes in a low pressure of a carbonaceous vapor, such as anthracene or acetylene, whereby .the incandescent filament decomposes the hydrocarbon vapor which comes in contact therewith and'efl'ects a chemical combination between the filamentary material and the carbon, probably forming tungsten carbide (W' C). During a subsequent heat treatment, the thorium oxide is reduced to metallic form by the carbide. A

Difliculty has resulted from this-carbonization process, due to the non-uniformity and spotty nature of the carbonization produced. The lack of uniformity of carbonization is caused by the difference in temperature of the various portions of the filament. The end portions of the filament are at a. much lower tempera ure than the medial portion, due to conduction of heat by the filament sup-.

ports. Due to this difference in temperature, if the average temperature of the filament is chosen so as to produce the mo st efl'ective carbonization, the end portions will be at too low a'temperature'to become efiectively carboniz'ed, whereas the central portion may be heated to a temperature above the dissociatlon point of the tungsten and carbon. As a consequence, the maximum carbonization will be produced at a point between the center' and each end of the filament.

Spotty or excess local carbon'ization is 7 brought about my irregularities in the cross section of the filame t. At points where the cross section is sma l, a higher temperature, known as hot spots, will be obtained,

causing the carbonization toproceed more.

rapidly at that point than at adjacent points.

Inasmuch as the resistance of the filament increases by carbonization, the carbonization at these hot spots usually builds up much more rapidly thanat the adjacent cooler portions, so that by the time the main body of the filament has become carbonized to a sufficient degree, there arenumerous' spots which are very much over-carbonized. This is a serious defect'since carbonization causes embrittlement' of. the filament, rendering it short lived and lowering its resistance to shock or vibration;

Another difficulty inherent in the methods employed heretofore, for carbonizing filaments, is the necessity of employing low pressures of the hydrocarbon vapor. The pressure of the hydrocarbon is quite critical, a slight variation in the pressure causing either over-carbonization or under-carbonization. It is very diflicult, however, to obtain accurately, a uniform low as pressure in the commercial application 0 the process and to avoid these difliculties it has been necessary to vary the time of carbonization in each instance, thus rendering it difficult to adapt .the process to automatic or purely mechanical operations.

One of the objects of the present invention is to avoid the above mentioned difiiculties and which will have a lon commercial life.

.A further object is to provide a suitable apparatus for carrying out the above objects.

Other objects and advantages will hereinafter appear. In accordance with the present invention the unactivated filaments are -sealed into an envelope in the usual manner and after the tubes have been exhausted, they are filled with a purified gas, such as hydrogen, which contains a vapor of a hydrocarbon. The

filaments are then flashed and allowed to cool in rapid succession so that the filament attains a temperature of about 2400 K. for a, fraction of a second only at each flashing. During the major portion of the interval while the filament is heating and cooling,

carbon is being deposited thereon, while carbide formation takes place only at the final temperature attained by the filament.

Due to the short period of each flash, there is very little loss of heat, due to radiation to the support wires and the filament is therefore heated substantially to the same temperature throughout its entire length. Oo n 25' sequently, the carbonization proceeds substantially uniform throughout the length of the filament. While the filament is being heated from approximately 200 or 300 K. to around 2200 K. carbon is being deposited thereon at a substantially uniform rate over the entire surface, since between these temperatures, carbon does not react with the tungsten and there is no carbideformation taking place... During the intervalv at which the filament is increasing from about 2200 to 24009 K. and again decreasing to 2200 K. the tungsten reacts with the carbon to form tungsten carbide. bon is already-present on the filament in a uniform deposit, when the filament attains fully understood, reference will be had to the accompanying. drawing in which the single figure illustrates 1n somewhat dlagrammatic form, apparatus for carrying out the present D r I {which the filaments are SllbJBCtBd' depends,

invention. v j The apparatus shown comprises a mainfold 1 to which are attached a number of electron discharge devices" 2 having sealed therein, filaments 3 which .it isdesired to activate. The discharge devices 2 are joined --to the manifold 1 by exhaust tubes 4. A vacuum pump (not shown) isjoined tothe manifold 4 1 by a conduit. -5'through a two-way valve'6.

The valve 6 also communicates, through con- However, since the car duits 7 and 8 with a" receptacle 9 containing for instance, ice water. \Hydrogen, from a line 11, is bubbled through the benzene 9 and saturated with the benzene vapor at the temperature of the cooling medium 10. A reservoir 12 is provided for storage of the carbon vapor containing hydrogen. A reservoir pump (not shown) for exhausting the reservoir 12 prior to emitting the hydrogen thereto, is joined to the reservoir by a connection 13 controlled by a valve 14. If desired, the reservoir may be omitted and the gas led directly from .the mixing chamber 9 to the manifold 1. 4

In order to flash .the filaments 3 intermittently, the leading-in wires 15 and 16 for each of the filaments are connected through a resistance 17 and a make-and-break device 18 with a source of current 19 The makeand-break device 18 is operated intermittently by a cam 20 driven by any suitable motor 21.

The operation of the apparatus is as follows. After the apparatus is sealed to the exhaust manifold 1 the valve 6 is adjusted so as to close off the hydrogen-line 7 and open the exhaust pump connection- 5 so as to exhaust the tubes. At the same time, the res-- .ervoir 12 may be exhausted by the reservoir pump through the valve 14. After the envelopes 2 have been exhausted, hydrogen bubbled through the benzene 9 is admitted to the envelopes at approximately atmospheric pressure from the reservoir 12, and

during which the contact device 18 is closed.

The frequency of the flashing may be varied within wide limits but I have obtained excellent results flashing the filament about once each second. The number of flashes to of course', upon the size of the filament, the maximum temperature thereof, the duration of the flash, and'the pressure or temperature of the hydrocarbon vapor. At the higher maximum' filament temperatures, fewer the motor 21 operated to flash the filaments flashes will be required, as will also be the' case when the partial pressure of the hydrocarbon is increased. As a specific example, employing benzene vapor mixed at a temper the hydrocarbon taken up by will be suificiently constant and such that.

drocarbon vapor admitted to the envelope,

and secondly, as a heat transferring medium for rapidly cooling the filament after each flash. By maintaining thetemperature of the benzene uniform, the partial" pressure of when the hydrogen is employed at approximately atmospheric pressure, it will contain a sufficient quantity of the hydrocarbon vapor to effect the carbonization of the filament.

This process is particularly well adapted for automatic carbonization'since a uniform pressure of the hydrocarbon may be easily obtained and maintained; and, by virtue of the intermittent flashing of the filament, the quantity of hydrocarbon obtained in the envelope is not critical and substantially uniform results may be obtained with widely varying'partial pressures of the hydrocarbon. It is possible, therefore, to obtain, within permissible limits, the same degree of carbonization with all filaments, simply by flashing them in the hydrocarbon vapor the proper number of times. Variations in original resistance from one filament to another are made negligible by employing a sufliciently high voltage and external series resistance. .-While' I have described a preferred embodiment of my invention it is to be understood that many modifications and changes may be made therein without departing from the invention and I do not desire to be limited except by the appended claims. What is claimed is:

1. Method of activating thoriated. tungsten filaments comprising iutermittentl 1ncandescing said filaments in a carbon earing atmosphere.

2. Method of activating thoriated tung. sten filaments comprising momentarily an intermittently heating said filaments to a carbonizing temperature in a carbon bearing atmosphere until the desired carbonization is effected.

3. Method of activating thoriated tungsten filaments com rising subjecting the filaments to num r of high temperature flashes of short duration in a carbon hearing atmosphere.

4. Methodof activating thoriated tungsten filaments comprising repeatedly heating the filaments momentarily to a temperature of about 2400 K. in the vapor of a hydrocarbon.

5. Method the hydrogenof activating thoriated tungsten filaments comprising repeatedly heating the filaments for a fraction of asecond to a temperature of about 2400 K. in a gas containing a carbonaceous material.

6. Method of activating thoriated tungsten filaments comprising heating the filaments to incandescence in a gaseous atmospherecontaining a small pressure of a vapor of a carbonaceous material.

7. Method of activating thoriated tungsten filaments comprising alternately depositing carbon substantially uniformly thereon and heating to a sufiiciently high temperature to cause a reaction between said carbon and the filaments.

8. Method of activating thoriated tungsten filaments comprising alternately heating the filaments'in a carbon vapor at a tem-' perature sufficiently high to effect a deposition of carbon thereon, without carbonization of the filament, and at a temperature sufliciently high to :eifect such carbonization.

9. Method of activating thoriated tungsten filaments comprising heatingthe filaments to below about 2200' K. in a "carbon bearing atmosphere to deposit carbon thereon and increasing the temperature of the filament above about 2200 K. to eflect a com-- bination of the carbon with the filament material.

10. Method of activating thoriated tungsten filaments comprising heating the filaments to below about 2200 K. in a carbon bearing atmosphere to deposit carbon there-. on and increasing the temperature of the filament above about 2200 K. to eifect a combination of the carbon with,the filament material, said latter temperature being maintained only for a brief interval. 11. Apparatus for carbonizing filaments comprising an enclosure for the filaments, means for supplying a carbon bearing atmosphere thereto, and means for intermittently and momentarily supplying a source of current to the filaments to heat the same to incandescence.

12. Apparatus for carbonizing filaments 1 comprising an enclosure for the filaments,

means for evacuating the enclosure and sup plying a carbon bearing atmosphere thereto, ,and interml'ttently operated means for momentarily incandesclng the filament.

.to said filament.

In testimony whereof, I have hereunto subscribed my name this 31st day of'March, 1927.

GEORGE DEAN ONEILL. 

